Date: Thu, 7 Oct 2021 10:11:20 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Amul Sul <sulamul@gmail.com>
Cc: "Bossart, Nathan" <bossartn@amazon.com>,
	Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	"andres@anarazel.de" <andres@anarazel.de>,
	"masao.fujii@oss.nttdata.com" <masao.fujii@oss.nttdata.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>,
	"mengjuan.cmj@alibaba-inc.com" <mengjuan.cmj@alibaba-inc.com>,
	"Jakub.Wartak@tomtom.com" <Jakub.Wartak@tomtom.com>,
	Ryo Matsumura <matsumura.ryo@fujitsu.com>
Subject: Re: prevent immature WAL streaming
Message-ID: <202110071311.4davhm2vralb@alvherre.pgsql>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: 
 <CAAJ_b97=34F9V99rO1RyGbKaUzyPiefeCWwa8U4pm2E2hZmKwA@mail.gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/202110071311.4davhm2vralb%40alvherre.pgsql>
Precedence: bulk

On 2021-Oct-07, Amul Sul wrote:

> While reading this commit (ff9f111bce24), wondered can't we skip
> missingContrecPtr global variable declaration and calculate that from
> abortedRecPtr value whenever it needed. IIUC, missingContrecPtr is the
> next page to the page that abortedRecPtr contain and that can be
> calculated as "abortedRecPtr + (XLOG_BLCKSZ - (abortedRecPtr %
> XLOG_BLCKSZ))", thoughts? Please correct me if I'm missing something,
> thanks.

I don't think that works -- what if the missing record is not on the
next page but on some future one?  Imagine an enormously large record
that starts on segment 1, covers all of segment 2 and ends in segment 3.
We could have flushed segment 2 already, so with your idea we would skip
ahead only to that position, but really we need to skip all the way to
the first page of segment 3.

This is easier to imagine if you set wal segment size to 1 MB, but it is
possible with the default segment size too, since commit records can be
arbitrarily large, and "logical message" records as well.

-- 
Álvaro Herrera              Valdivia, Chile  —  https://www.EnterpriseDB.com/